1. Field of the Invention
The present invention relates to a die casting mold in which a recessed cavity portion is provided on an insert, and relates to a method of manufacturing and casting the die casting mold.
2. Description of the Related Art
A conventional die casting mold of the above-described type is disclosed, for example, in JP-B-Hei 7-73783. The mold described in this publication includes a fixed mold attached to a fixed platen of a die casting machine, and a movable mold attached to a movable platen thereof.
The fixed mold includes a fixed insert, on which a recessed portion defining a cavity is provided, and a mold body for holding the fixed insert. The movable mold includes a movable insert that has a projecting portion facing the recessed portion which forms the cavity in cooperation with the fixed insert, and a mold body for holding the movable insert.
As described above, the mold which is divided into the insert and the mold body for defining the cavity allows the optimum selection of metal materials, types of heat processing to enhance the hardness, and so forth, for the functions of the mold members. In other words, the aforementioned construction is used in order to provide an insert of a metal material having sufficient hardness and toughness to withstand the die casting environment, and allow the insert to be subjected to heat processing so that the insert has a hardness greater than that of the mold body.
This type of insert is used to repeat shots several thousand times in die casting operations, which causes a tendency for a cast product to have defects on the surface, which are called heat checks.
The heat checks refer to a lattice pattern (check pattern) on a portion of an outside surface of the cast product whose curvature is relatively large.
The cause of heat checks is that the lattice-patterned cracks are produced on the inner surface of the recessed cavity portion of the insert. Molten metal runs through and solidifies in the cracks of the mold, and protrudes from the surface of the cast product as heat checks.
Cracks, a cause of the heat checks, are produced on the surface of the mold by repeated heating and cooling due to thermal stresses. More specifically, cracks are produced due to thermal stresses that concentrate on a portion of the inner surface of the recessed cavity portion whose curvature is relatively large. Such cracks are initially shallow and short. Repeated thermal expansion/contraction of the mold results in the fine cracks gradually becoming larger (deeper and longer) to permit molten metal to easily run into the cracks.
In the event that the heat checks are produced on the exterior part of the cast product, in the conventional manner, a grinding tool, such as sandpaper and a buffer, has been used to remove the heat checks.
In addition, in the event that it takes a longer time to repair the cast products, or the repair work is more difficult, due to larger heat checks or the increased number of locations where heat checks occur, the insert is repaired to eliminate the cracks or is replaced with a new one by a method to be discussed below.
The insert is repaired by either one of the two types of methods below. The first method includes removing a portion of the insert where cracks, a cause of heat checks, are produced; build-up welding on this portion; and reprocessing thereof, thereby restoring the mold shape. The second method includes interposing a spacer between the inner bottom of the recess of the mold body and the insert fitted into the recess.
Interposing the spacer between the insert and the mold body in such a manner causes the entire insert to protrude from the mold body by an amount consistent with the thickness of the spacer. The second method also includes removing a mating surface of the insert to the other mold and a surface of the recessed cavity portion by means of electrical discharge machining or the like by an amount of the protrusion of the insert from the mold body. Thereby, the mold is restored to the original shape.
Among conventional inserts for die casting molds, an insert to be used for casting large parts, such as a vehicle frame for a motorcycle, has large dimensions including a thickness sufficient to be almost equal to its height or width. Even if such a large-dimensioned insert is made of the optimum material and subjected to heat processing for enhanced hardness, there are still problems with heat checks that can occur in a relatively short time after the heat processing.
The potential cause of this is that the insert is not always subjected to heat processing entirely equally due to its relatively large size and volume, and accordingly, the cavity is not equally subjected to the heat processing.
In addition, a worker has to manually repair a cast product having heat checks or remove the heat checks. Moreover, the worker must be careful during this repair work because it involves machining of the exterior portion of the product. This creates a problem of a significant increase in man-hours for removing the heat checks.
Further, even if the cracks on the inside of the recessed cavity portion, which can cause heat checks, are removed, there still arises a problem that the heat checks can re-occur in a relatively short time after the repair. The reason for this is that the insert is repaired by removal of the cracks on the inside of the recessed cavity portion, and then build-up welding the cracked portion, which results in lower hardness and toughness on the build-up welded area compared to the other areas. In contrast, the method, using the spacer to allow the large-dimensioned insert to protrude from the mold body and remove the protruding portion, fails to completely remove deep cracks largely due to the long time that elapses before the repair. This prevents the insert from being restored to its initial conditions.
Therefore, the remaining minor cracks on the insert spread, causing heat checks in a short time after their removal. As described above, the method of removing the protruding portion of the insert from the mold body must involve cutting a peripheral portion of the insert around the cavity. A sprue or other portions tend to be provided around the peripheral portion.
In other words, the removal work must involve cutting not only the protruding portion of the insert, but also the peripheral portion thereof. This creates a problem of a longer time required for the cutting work.
In addition, in the case of replacing the insert with a new one, rather than refurbishing the insert, other usable portions of the cavity, such as the sprue portion, are also covered by this replacing, which results in an increased cost.